#include<bits/stdc++.h>
#include<atcoder/all>
using namespace std;
using namespace atcoder;
#define OVERLOAD_REP(_1, _2, _3, name, ...) name
#define REP1(i, n) for (auto i = std::decay_t<decltype(n)>{}; (i) != (n); ++(i))
#define REP2(i, l, r) for (auto i = (l); (i) != (r); ++(i))
#define rep(...) OVERLOAD_REP(__VA_ARGS__, REP2, REP1)(__VA_ARGS__)
#define REP(i, l, r) rep(i, l, r+1)
#define all(x) (x).begin(),(x).end()
#define rall(x) (x).rbegin(),(x).rend()
using ll = long long;
using ull = unsigned long long;
using ld = long double;
using P = pair<ll,ll>;
struct Edge {
  int to; char l;
};
//using Graph = vector<vector<int> >;
using Graph = vector<vector<Edge> >;
const ull INF = 2e18;
//const int INF = 2e9;
template<class T> using vc = vector<T>;
template<class T> using vv = vector<vector<T> >;
template<class T> using vvv = vector<vector<vector<T> > >;
template<class T> using pq = priority_queue<T>;
template<class T> using pq_g = priority_queue<T, vc<T>, greater<T> >;
template<class T> istream& operator>>(istream& i, vc<T>& v) { rep(j, 0, v.size()) i>>v[j]; return i; }
template<class T> ostream& operator<<(ostream& o, vc<T>& v) { rep(j, 0, v.size()) o<<v[j]<<" "; return o; }
template<class T> bool chmin(T& a, T b) {
  if(a > b) {
      a = b;
      return true;
  }
  return false;
}
template<class T> bool chmax(T& a, T b) {
  if(a < b) {
      a = b;
      return true;
  }
  return false;
}

int N;
int main() {
  // 高速化
  ios::sync_with_stdio(false);
  cin.tie(nullptr);

  // 小数点の出力桁数を指定
  cout << fixed << setprecision(10);
  // メイン
  ull K;
  cin >> N >> K;
  string S;
  cin >> S;
  S += ' ';

  Graph G(N+1);
  rep(i, 0, N) {
    char n = S[i]-'0';
    if(1 <= n && n <= 26 && S[i+1]!='0') {
      G[i].push_back({i+1, 'a'+n-1});
    }
    if(i != N-1) {
      string s = {S[i], S[i+1]};
      char n = stoi(s);
      if(1 <= n && n <= 26 && S[i+2]!='0') {
        G[i].push_back({i+2, 'a'+n-1});
      }
    }
  }
  vc<ull> cnt(N+1);
  cnt[N] = 1;
  for(int i = N-1; i >= 0; i--) {
    cnt[i] = 0;
    for(auto [j, _] : G[i]) cnt[i] += cnt[j];
    chmin(cnt[i], INF);
  }

  K--;
  string res = "";
  auto rec = [&](auto& rec, int v) {
    if(v==N) return;
    if(G[v].size()==1) {
      res += G[v][0].l;
      rec(rec, G[v][0].to);
    } else {
      if(cnt[G[v][0].to] > K) {
        res += G[v][0].l;
        rec(rec, G[v][0].to);
      } else {
        res += G[v][1].l;
        K -= cnt[G[v][0].to];
        rec(rec, G[v][1].to);
      }
    }
  };
  rec(rec, 0);
  cout << res << endl;

  return 0;
}